Endometriosis, the third leading cause of gynecologic hospitalization in the United States, remains one of the most enigmatic gynecologic pathologies. Endometriosis is defined as the presence of endometrial tissue outside of the uterine cavity. These implants respond to the hormonal cues of the menstrual cycle and "bleed" as they would in the uterus. The consequence is the development of adhesions, scarring, and painful inflammation. Signs and symptoms include dysmenorrhea, dyspareunia, infertility, dysuria, and irritable bowel syndrome.The effects of the disease can be physically and mentally debilitating with frequent misdiagnoses and poor treatment options. Its prevalence among U.S. women has been estimated to be approximately 10%, [the] time from onset of symptoms to laparoscopically confirmed diagnosis is estimated to average between 6 and 11 years. To date, the etiology of endometriosis remains unknown and few epidemiologic studies exist. Using data on 2,690 laparoscopically confirmed incident cases of endometriosis collected from the Nurses' Health Study II, an ongoing, prospective cohort study that began in 1989, the applicant proposes a study to assess the following hypotheses: a) Women with menstrual characteristics of younger age at menarche, longer time to menstrual regularity, or shorter menstrual cycle length are at higher risk of endometriosis. b) Women with a low waist-to-hip ratio are a higher risk of endometriosis. c) Women with a higher body mass index at age 18 are at lower risk of endometriosis. d)Women who were born with a greater birthweight are at higher risk of endometriosis. All analyses will control for other known and suggested risk factors for endometriosis such as oral contraceptive use and cigarette smoking. The applicant will have more than 90% power to evaluate the above hypotheses. These analyses will be the first prospective data with adequate power to evaluate this important and understudied cause of morbidity among premenopausal women.